Method of purification of a redox mediator before electrolytic regeneration thereof

ABSTRACT

A method for purifying a Redox mediator used in a chemical process of oxidation of organic compounds. The desired purification is obtained by recovering the mediator in the form of a solution and heating this solution to evaporate the volatile impurities contained in it and to oxidize the non volatile impurities into compounds which precipitate and are extracted by filtration. The purification takes place before the mediator is regenerated in the electrolysis cell. This prevents the impurities contained in the mediator solution to negatively affect the operation of this cell.

FIELD OF THE INVENTION

[0001] The invention relates to a method for purifying a Redox mediatorbefore electrolytic regeneration thereof during a chemical oxidationprocess of organic compounds.

DESCRIPTION OF PRIOR ART

[0002] It is well known that oxidation of unsaturated long chain fattyacids such as oleic acid, yields short chain fatty diacids, such asazelaic acid or pelargonic acid. Usually, such an oxidation is carriedout with a solution of chromic (Cr VI) acid and sulfuric acid producedby electrolysis of an aqueous solution of chromium (Cr III) sulphate andsulfuric acid. During the electrolysis, hydrogen is released at thecathode and the chromium sulfate is converted into chromic acid andsulfuric acid at the anode (see U.S. Pat. No. 2,450,858 granted in1948).

[0003] When this type of oxidation (also called indirectelectrosynthesis) is used, it is customary to recover the solution ofchromium sulfate and sulfuric acid obtained after the oxidationreaction. The so-recovered solution is then recycled towards anelectrolysis cell in order to regenerate it to obtain the desired ionicspecies for the oxidation reaction.

[0004] A major difficulty of this process of recovering and regenerationof the electrolyte lies in the presence of organic impurities in theelectrolytic solution. It turns out that these impurities accumulate anddeposit on the electrodes of the electrolysis cell. This results in areduction of the electrolysis current efficiency.

[0005] It is also known that solutions of Cerium III/Cerium IV are usedfor the electrochemical oxidation of other types of organic products.Electrochemical oxidation of other types of organic products such as,but not limited to aromatic aldehydes and quinones as described byHARRISON in U.S. Pat. No. 5,296,107 and by KREH et al in U.S. Pat. Nos.4,639,298; 4,647,349; 4,620,108; 4,701,245 and 4,794,172. The commercialapplicability has been described by HARRISON in Journal of new Materialsfor Electrochemical Systems, January 1999.

[0006] Work done by the LTEE in collaboration with W. R. Grace & Co. hasled to the development of a generic technology for the selectivemanufacture of a whole array of chemical products of high interest. Thesuccess of this technology has already been confirmed during pilotprojects directed to the synthesis of anthraquinone, aminoanthraquinoneand para-tolualdehyde.

[0007] However, even in these cases, the presence of soluble organiccompounds in the mediator solution recovered at the end of the reactionconsiderably affects the regeneration efficiency of the catalyst in anelectrolytic cell. More precisely, these compounds affect the currentefficiency and the lifetime of the electrodes and they generatedrawbacks of operation in continuous mode. It turns out that thepresence of organic compounds can block the surfaces of the electrodesand therefore reduce the production rate of the Redox mediator. Tomaintain this rate, the current must be increased.

[0008] In light of the above, it is therefore obvious that it isessential to purify the electrolytic solution recovered after theoxidation reaction before regenerating it.

[0009] In order to do it, it has already been suggested to treat themediator solution with activated charcoal to absorb the dissolvedorganic molecules present in it. Although this method seems to work, ithas a number of drawbacks.

[0010] First of all, it is costly because the consumption of activatedcharcoal expressed in kg per kg of obtained product is high (this is adirect consequence of the low electrolyte concentration and therefore ofthe high V_(electrolyte)/kg_(product) ratio).

[0011] Secondly, after use, the activated charcoal must either bediscarded or regenerated, but before doing so, the redox mediator mustbe recovered for economic and environmental reasons. Such can be done bywashing the activated charcoal with water. However, concentrating therecovered redox reagent by evaporation is energy intensive. Handling ofwet activated charcoal is also a labour intensive activity (this is atime consuming operation that results in higher labor costs andproduction costs).

[0012] Thirdly, this method cannot be used on non organic molecules andnon-aromatic products having a low affinity for activated charcoal.

SUMMARY OF THE INVENTION

[0013] The object of the invention is to solve the above-mentionedproblem that occurs in all the chemical processes that useelectrochemically regenerated Redox couples.

[0014] More precisely, the object of the present invention is to providea method that is both simple and efficient to solve the problem that waspreviously mentioned. This method essentially consists of purifying thesolution containing the Redox mediator that is recovered from theoxidation reactor before introducing it into the electrolysis cell wherethe regeneration takes place. This purifying step is of a greatimportance since it allows removal of organic impurities that tend todeposit on the electrodes or consume regenerated mediator if they arenot extracted from the electrolytic solution and which can thereforeaffect the operation of the electrolysis cell and the efficiency of theregeneration.

[0015] Thus, the method according to the invention is devised to purifya Redox mediator used in a chemical oxidation process of organiccompounds before this mediator is regenerated electrochemically in anelectrolysis cell. This method comprises the steps of:

[0016] recovering the mediator in the form of a solution containingvolatile, soluble and insoluble impurities;

[0017] subjecting the solution containing the mediator to a thermaltreatment, this thermal treatment being carried out in a reactor kept ata temperature high enough to allow oxidation of the impurities by meansof left-over unused mediator that is still present in the solution underits oxidative form, or by addition of a given amount of said regeneratedmediator and simultaneously to allow elimination of the volatileimpurities present in the solution; and

[0018] filtering the solution that contains the mediator, to remove theinsoluble impurities therefrom.

[0019] This series of steps constitutes the heart of the invention sinceit considerably increases the technical and economical feasability ofthe whole chemical process.

[0020] In this connection, it is worth mentioning that this method forpurifying an electrolyte, is generic, economical and compatible with allthe processes used for the treatment of an electrolyte in view ofefficiently recycling it.

DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic diagram of a process for the chemicaloxidation of organic compounds which incorporates the method accordingto the invention;

[0022]FIG. 2 is a graph showing the Ce IV consumption in the reactorused for the thermal treatment, as a function of the time and ofdifferent excess quantities of Ce IV added at a 60° C. temperatureduring 100 minutes when the method of purification according to theinvention is used in a process for the oxidation of p-xylene top-tolualdehyde;

[0023]FIG. 3 is a graph showing the amount of p-tolualdehyde in thereactor as a function of the time and of different excess quantities ofCe IV added under the same conditions as in FIG. 2;

[0024]FIG. 4 is a graph showing the amount of toluic acid in the reactoras a function of the time and of different concentrations of Ce IV addedunder the same conditions as in FIG. 2;

[0025]FIG. 5 is a graph showing the amount of terephtaldehyde in thereactor as a function of the time and of different concentrations of CeIV added under the same conditions as in FIG. 2;

[0026]FIG. 6 is a graph showing the amount of carboxybenzaldehyde in thereactor as a function of the time and of different concentrations of CeIV added under the same conditions as in FIG. 2;

[0027]FIG. 7 is a graph showing the Ce IV consumption (in %) in thereactor as a function of the time, with an excess quantity of added CeIV of 10% and at different temperatures for 100 minutes, when use ismade of the purifying method according to the invention;

[0028]FIG. 8 is a graph showing the amount of p-tolualdehyde in thereactor as a function of the time under the same conditions as in FIG.7;

[0029]FIG. 9 is a graph showing the amount of toluic acid in the reactoras a function of the time under the same conditions as in FIG. 7;

[0030]FIG. 10 is a graph showing the amount of terephtaldehyde in thereactor as a function of the time under the same conditions as in FIG.7; and

[0031]FIG. 11 is a graph showing the current efficiency of foursolutions, viz one that is purified with charcoal, another one that iscontaminated and two others that underwent a similar thermal treatmentfor 30 minutes at 100° C. with an addition of 10% of Ce IV.

DETAILED DESCRIPTION OF THE INVENTION

[0032] As previously mentioned, the method according to the invention isintended to be used for purifying a Redox mediator before regeneratingit in an electrolysis cell.

[0033] As non-restrictive examples of Redox mediators, reference can bemade to Ce III/Ce IV mediator, Cr IV/Cr IV mediator and a mixturethereof.

[0034] The Ce III/Ce IV mediator can be used in the following processfor the synthesis of:

[0035] p-tolualdehyde starting from p-xylene;

[0036] naphtoquinone and its derivatives starting from naphtalene andits derivatives; or

[0037] chlorobenzoquinone starting from chloroaniline.

[0038] The Cr III/Cr IV mediator or a mixture of Ce III/Ce IV and CrIII/Cr IV can be used for the synthesis of azelaic and pelargonic acidsstarting from oleic acid,

[0039] The above mentioned examples of processes are given forillustrative purposes only. In fact, there are numerous other Redoxcouples with similar possible uses.

[0040] The first step of the method according to the invention isconventional and is presently already in use. It consists of recoveringthe electrolytic mediator to be regenerated in the form of a solution.Coming out of the reactor, this solution usually contains volatile,soluble and insoluble impurities along with the catalyst, a small amountof which is still active.

[0041] The second step consists in subjecting the recovered solution toa thermal treatment. This treatment is carried out in a reactor that iskept at a temperature that is high enough to allow oxidation of theimpurities by means of the left-over unused mediator that is almostalways present in the solution. This thermal treatment also permits toeliminate volatile impurities. In practice, in the case of theabove-mentioned processes listed as examples, the reactor can be kept ata temperature ranging between 40 and 120° C., preferably 70 and 110° C.,and more preferably between 90 and 100°0 C.

[0042] The third and last step consists of filtering the solution inorder to remove the insoluble impurities contained therein, before it isfed to the electrolysis cell.

[0043] In the event that the left-over unused oxidation mediator presentin the recovered solution is insufficient to obtain the desiredtreatment it is possible to add to the solution a given amount of themediator regenerated in the electrolysis cell, as illustrated in FIG. 1.Therefore, for example, in the case illustrated in FIG. 1, 10% of theregenerated mediator (Ce IV) is added to the solution in order toincrease the amount of active mediator (Ce IV) present in the recoveredsolution, which in this case is only 5%. Of course, the amount ofregenerated mediator to be added may substantially vary, since itdepends on the parameters of the chemical reaction and of the requiredneed for an oxidizing agent, to eliminate the impurities present in thesolution.

[0044] As can be understood, the method according to the inventionallows for maximum oxidation of the organic by-products present in theelectrolyte. As aforesaid, this oxidative step is carried out at hightemperature in the presence of the leftover mediator and/or ofregenerated mediator added to the solution.

[0045] In practice, the method according to the invention has severaladvantages.

[0046] First of all, the organic molecules become inert, under theirmost oxidized form and they can no longer react with the electrolytethat is produced in the electrolysis cell. Control on the currentefficiency can therefore be obtained more easily.

[0047] Secondly, oxidation of most of the organic molecules results inthe formation of organic acids. These organic acids have very lowsolubilities and precipitate as solids. The subsequent filtration thatis preferably tangential, permits to extract these acids from theelectrolytic medium and therefore to lower the organic load of theelectrolyte. Therefore, the lifetime of the electrodes is less affecteddue to the fact that the electrolyte is purer.

[0048] Thirdly, the high temperature permits to evaporate any residualsolvent as well as all the volatile organic molecules without evenhaving to subject them to an oxidation. The so evaporated leftoversolvent can be recycled in the process.

[0049] Fourthly, this method which actually amounts to a<<stabilization>> by oxidation of the recovered solution, can be appliedto all aromatic and aliphatic organic molecules.

[0050] Thus, the method according to the invention has the advantages ofbeing generic, effective and functional for the treatment of theelectrolytes used in chemical processes of oxidation also known as“indirect electrosynthesis”. The thermal treatment of the usedelectrolyte is the key feature of the method according to the invention.This thermal treatment is simple and very advantageous since it does notrequire any additional equipment. In fact, it only requires an increasein the capacity of the electrochemical cell.

[0051] As previously mentioned, one can add a given amount of mediatorregenerated in the electrolysis cell directly to the reactor where thepurification takes place to help completion of the oxidation of thesolution containing the mediator and the impurities. In other words,part of the regenerated electrolytic solution can be used solely for theoxidation of the used mediator solution.

[0052] It is important to mention that, during tests carried out by theApplicant, the method according to the invention was only subjected toone recycling. However, it could be expected that a greater number ofrecycling, for example 30 or more, would help to increase the efficiencyof the treatment and verify that the efficiency of the regenerationcurrent and the amount of residual organic products in the electrolyte,stabilize after a given number of recyclings.

[0053] The following examples illustrate the invention. These examplesare non-restrictive and given only for informative purposes.

EXAMPLE 1 Application of the Method According to the Invention in aProcess of Synthesis of p-tolualdehyde

[0054] It is known that the synthesis of p-tolualdehyde is carried outby oxidation of p-xylene in the presence of Ce (IV) that can beregenerated electrochemically at the anode of an electrolysis cell (seereaction 1 below). In that process, the starting material, viz.p-xylene, is oxidized with cerium (IV) in a chemical reactor understirring to the corresponding aldehyde, p-tolualdehyde (PTA).

Ce(CH₃SO₃)₃+CH₃SO₃H→Ce(CH₃SO₃)₄+H⁺+e⁻  (1)

[0055] By-products are also produced during this synthesis. Theby-products result from an under-oxidation or over-oxidation of PTA(alcohols, acids, . . . ).

[0056] After the chemical reaction is completed, the organic moleculeslisted above are present at different concentrations in the recoveredsolution containing the electrolyte. During the tests carried out by theApplicant, chemical yields of 85-90% of PTA were obtained. The majorby-products were p-tolu{umlaut over (n)}ic acid (10-14%) and terephtalicacid (1-2%). A large amount of the p-toluïc acid can be removed byfiltration after the chemical reaction. However, tests carried out in apilot plant have shown that the p-toluïc acid represents approximately5% of the formed products and therefore that the amount of p-toluïc acidin precipitated form is practically negligible. Three (3) consecutiveextractions with cyclohexane helped to eliminate 80% of the p-toluïcacid in the acidic phase (residual concentration of 130 ppm). Theresidual concentration of said acid in the anolyte reservoir (70-80 ppm)was significantly reduced after treatment of the acidic phase with anactivated charcoal (10 ppm, trial).

[0057] If the electrolyte is not purified, the above-mentioned moleculeswill react either at the electrode where the electrolyte is regenerated,or with the Ce IV regenerated at that electrode. These reactions willthen generate a decrease in current efficiency. In the case where theby-products may adsorb on the surfaces of the electrodes, a current drop(or a voltage increase in galvanostatic mode) can be observed. Thisphenomenon is due to a decrease of the active surfaces of theelectrodes. In this case, cleaning of the electrodes is necessary. Insome cases, the presence of organic molecules can even affect the lifespan of the electrodes. For example, the life span of lead electrodescan be severely affected in the presence of acetic acid produced byoxydation of organic molecules.

[0058] The method according to the invention permits to <<stabilize>>these molecules by oxidation at high temperature in the presence of anexcess of oxidant. This method allows a better control on the operationparameters (intensity, current efficiency). Moreover, it permits in somecases, to extract from the medium, a greater amount of the organic loadby filtration. Preferably, a tangential filtration will be used in orderto extract the solid organic particles in suspension in the solution.Such a use is important, since the solid particles to be extracted areusually of colloidal size and may clog the pores of conventionalfilters.

[0059] Thermal treatment produces highly oxidated molecules which aresolid for the most part. Since these products are insoluble in theaqueous acid phase, they precipitate in the form of very fine particlesthat can be separated from the solution by tangential filtration.

[0060] This control on the operation parameters is extremely importantand constitutes one of the most notable advantages of the method ofpurification according to the invention. The organic molecules react ina forced and rapid manner (due to the elevated temperature) outside ofthe electrochemical cell with the oxidized contaminants being separatedbefore affecting electrode performance instead of reacting inside theelectrochemical cell and affecting the electrode performance causing anuncontrollable drop in current efficiency (due to the consumption ofregenerated mediator and the formation of deposits on the electrodesurface).

[0061] During the tests carried out by the inventors, the followingequipments were used:

[0062] Testing bench:

[0063] equipped glass chemical reactors of 20 l and 2 l

[0064] Ce electrochemical cells with 40 l and 2 l capacity

[0065] Chemical analysis apparatus:

[0066] automatic titrators,

[0067] CPV and HPLC chromatographs for sample analysis.

[0068] The anolyte used in the tests consisted of a solution that waspurified over actived charcoal. It was not a contaminated solution. Tocarry out the chemical reaction of oxidation, 50 liters of this solutionwere used with a concentration of 0,4 M of Ce IV. Table 1 characterizesthe <<clean>> solution obtained before and after regeneration. TABLE 1Characterization of the cerium solution considered as “clean” (referencesolution) Before After regeneration Regeneration Ce (IV) (M) 0.004 0.436Total Ce (M) 0.746 0.723 Acidity AMS (M) 4.22 4.38 Benzenedimethanol(ppm) ND ND Terephtalic acid (ppm) ND ND Carboxybenzaldehyde (ppm) ND NDTerephtaldehyde (ppm) ND ND 4-methylbenzylalcohol ND ND (ppm) ToluicAcid (ppm) ND ND PTA (ppm) ND ND p-Xylene (ppm) ND ND Unknown 3.85 min(ppm) ND ND Unknown 7.32 min (ppm) ND ND

[0069] Reaction

[0070] The synthesis reaction was carried out with equipment made upprimarily of glass, stainless steel and Teflon®. As previouslyindicated, the equipment was composed of a 20 l reactor with athermocouple, a reflux condenser and a mixer. The oxidative solution hada concentration of Ce IV of about 0,4 M and a total cerium concentrationof about 0,75 M. This solution was acidified with about 4 M inmethanesulfonic acid (CH₃SO₃H). During the tests 16 l of this solutionwere necessary to carry out the reaction. The volume of reactant(solution to be oxidized) was determined as a function of the Ce IVconcentration in the oxidative solution.

[0071] The reactant was added to the oxidative solution at a temperatureof 60° C. The reaction was carried out under stirring with a mechanicalstirrer for a predetermined time. The temperature was continuouslymonitored. At the end of the reaction, the stirring was stopped and theextraction step was perfomed as rapidly as possible.

[0072] The parameters of operation were the following:

[0073] T=60° C.,

[0074] Reaction time=10 min,

[0075] Molar ratio (Ce IV/p-xylene)=4.5/1,

[0076] Electrolyte acidity=4.5 MSA,

[0077] Ce (total)=0,75 M and Ce IV=0,4 M.

[0078] After the chemical reaction, two consecutive extractions werecarried out with p-xylene (starting material) at ambiant temperature.

[0079] The used electrolyte recovered in this way will hereafter becalled the <<contaminated>> solution.

[0080] The contaminated solution having a lower amount of active ceriumobtained after the reaction was then regenerated in an electrolysiscell. The set-up used in this case was the electrolysis set-up ofElectrocatalytic Ltd., modified to fit a FM01-LC cell of ICI. Thisequipment is designed to resist acids (the piping is mostly composed ofPVDF and Teflon®).

[0081] The contaminated solution containing mainly Ce III andmethanesulfonic acid (CH₃SO₃H) was brought to the anodic compartment ofthe cell by a system of pumps and piping. In the anodic compartment, anelectric current transformed Ce III into Ce IV up to a concentration ofapproximately 0.4 M. The electrolysis of the solution was done at 60° C.

[0082] By “cumulative efficiency of a solution”, it is meant thecapacity (in percentage) of a solution to transmit the current that isapplied to it in order to carry out a desired reaction. In other words,the cumulative efficiency is the ratio of the current used to carry outthe desired reaction to the applied current. In this example, theefficiency of the clean solution with a concentration of approximately0.4 M of Ce IV, was of 92%. This efficiency was considered as thereference efficiency for the non contaminated or “clean” solution inthis example. The efficiency of the thermal treatment of the electrolytewas therefore compared with a value of 92%, as will be seen hereafter.

[0083] Regeneration of the contaminated solution obtained with notreatment before its introduction into the electrolysis cell gave a newsolution having the lowest current efficiency value that can be obtainedfor an anolyte that has only undergone one cycle in the system. Thisvalue is considered as the lowest one since no treatment was carried outon the electrolyte. The value obtained for the final current efficiencywas 85%.

[0084] Thus, it appears that the presence of residual organic productsin the electrolyte after one cycle of chemical reaction decreases thecurrent efficiency by 5-7%. The purpose of the electrolyte treatmentcarried out by the method according to the invention is to increase thisefficiency to get a current efficiency close to the one of the “clean”solution.

[0085] Optimization of the treatment

[0086] In the thermal treatment, three factors are extremely important:the duration of treatment, the temperature of treatment and theconcentration of Ce (IV) in the recovered solution.

[0087] The latter factor, viz. the Ce (IV) concentration, must beminimized. Indeed, the amount of Ce (IV) solution to be added to themedium in order to carry out the oxidation of the impurities isproportional to the energetic consumption of the cell. The larger is theadded amount of Ce (IV), the higher are the energy costs for thetreatment. Moreover, the impact is not only on the operation costs butalso on the capital costs because additional electrochemical cells maybe necessary.

[0088] The advantage of the purification method according to theinvention lies in its simplicity because it does not require anyadditional equipment. It only requires enlargement of the existingequipment.

[0089] It should be noted that during the treatment, the residual Ce(IV)is lost due to the fact that activated charcoal column could still beused in the process for safety reasons and the fact that Ce (IV) reactswith activated charcoal.

[0090] Thus, it is essential to determine the minimum quantity of Ce(IV) that must be added to the used electrolyte to carry out theoxidation of the impurities.

[0091] The duration and temperature of treatment must also be selectedto have a short and efficient time of treatment. Proper temperatureselection permits to decrease the duration of treatment and to oxidizesome organic molecules that can not be oxidized at lower temperatures.

[0092] The thermal treatment was optimized in the case of p-tolualdehydewith these three variable parameters: duration of treatment, temperatureand amount of Ce (IV).

[0093] Ce (IV) Concentration (lower limit)

[0094] The object of this section consists of evaluating the minimumquantity of Ce (IV) to be added to the used electrolyte in order tofully oxidize the residual organic products present in it. To find thislower limit, the above-mentioned chemical reaction was carried out underthe conditions described previously.

[0095] After completion of the chemical reaction, two consecutiveextractions were carried out to extract the products of the reactionfrom the electrolyte. The obtained electrolyte was used as reference andis hereinafter called the “contaminated and untreated electrolyte”. Tothis contaminated electrolyte, 5%, 10%, 15% and 20% in volume of theinitial electrolyte having a concentration of about 0,4 M of Ce (IV)were added during the tests. Then, these solutions were heated understirring at T=60° C. for 100 minutes. This allows to determine theminimal quantity of Ce (IV) necessary to carry out the desired extensiveoxidation.

[0096] The amount of cerium as well as the amount of the reactionproducts in the solution were measured as a function of the time and theresults are reported in FIGS. 2 to 6. Table 2 shows the residual amountof Ce IV as a function of the added amount of Ce IV. TABLE 2Concentration of Ce (IV) as a function of the amount of Ce (IV) addedinitially Ce IV added initially 5% 10% 15% 20% Ce (IV) added (mmoles)18.48 39.09 60.24 82.87 Residual Ce (IV) (mmoles) 5.64 24.67 44.68 65.14Residual Ce IV % 30.5 63.1 74.7 78.6

[0097] FIGS. 2 to 6 illustrate the performance of the different amountsof Ce IV used in the tests in relation to the different chemicalproducts found in the electrolyte solution stemming from the oxidationprocess of p-tolualdehyde.

[0098] Based on the results that were so obtained, an addition of 10%would be a good compromise. This amount permits to leave a surplus ofcerium as a precautionary measure.

[0099] Effect of the temperature and duration of treatment

[0100] The next step was to optimize the temperature and the duration oftreatment. Obviously, these two parameters are linked, andtheoretically, the higher is the temperature, the shorter should be thereaction time. The object of this step was to find an interestingcompromise between the temperature of treatment and the duration of thetreatment. The treatment time is a critical factor for carrying out theprocess in a continuous mode since a long reaction time results in ahigher volume of electrolyte in circulation and the need for largerreactors.

[0101] Tests were carried out at different temperatures. A sampling wasperformed at precise intervals to determine the best duration for thistype of treatment in the case of the electrosynthesis of PTA. The testswere carried out with the addition of 10% of Ce IV to the electrolyteand at temperatures of treatment of 60, 70, 80, 90 and 100° C.,respectively.

[0102] The amount of Ce (IV) and the other products of reaction weremeasured as a function of the time at the different temperatures oftreatment and the results are reported in FIGS. 7 to 10.

[0103] An analysis of the obtained results shows that a temperature of100° C. and a time of 30 minutes are sufficient to achieve the desiredpurification. In fact, after about 20 minutes, there was no modificationin the amount of organic products in the solution. The only product thatrequired a longer time of treatment is toluic acid. Precautionarymeasures led to the selection of a time of 30 minutes. However, it isworth noting that temperatures higher than 100° C. were not studiedbecause they are very close to the boiling temperature of theelectrolyte. Treatment in a tank under pressure can be designed to reachhigher temperatures. However, since the differences observed between thetemperatures of 90 and 100° C. were small, such a treatment is probablynot necessary in view of the small increases in temperature (ex: 110°C.).

[0104] At 60° C., the oxidation of toluic acid to terephtalic acid (FIG.10) does not seem to be completed in the chosen time interval. In fact,the concentration increases in the solution during the studied timeinterval, whereas at temperatures higher than 70° C., a decrease in theconcentration was observed.

[0105] For the treatment of the electrolyte used for theelectrosynthesis of p-tolualdehyde, the following parameters were used:an addition of 10% (v/v) of a regenerated cerium solution with aconcentration of 0,4 M of Ce IV, a treatment temperature of 100° C. anda treatment time of 30 minutes. Obviously, the treatment time must beoptimized again when used on a large scale. Indeed, this time shoulddepend at least in part on the mixing conditions and the geometry of thereactor.

[0106]FIG. 11 shows that the selection of parameters was very well doneand that the current efficiency of the used solution after treatment wasas good as that of a clean solution. FIG. 11 also shows that thereproducibility is excellent since the results obtained for the twotreated solutions are almost identical.

EXAMPLE 2 Electrosynthesis of azelaic and pelargonic acids with CrIII/CrVI

[0107] The electrolyte used to mediate the oxidation of oleic acid inorder to synthesize pelargonic and azelaic acids is the Redox couple CrIII/Cr VI. A combination of Ce III/Ce IV and Cr III/Cr VI can also beused to mediate the oxidation of oleic acid. However, an electrolytemade up only of cerium will not work.

[0108] Untreated electrolyte (prior art)

[0109] With a <<new>>chromium solution used as a reference with noorganic matter contained therein, a current efficiency of 78% with 0,22M of Cr VI was obtained.

[0110] With the same solution after having been used once and not havingundergone any treatment other than extraction, filtration anddecantation (hereafter called <<used untreated solution>>), the currentefficiency dropped tremendously in comparison to the new solution. Forthe used untreated solution there was a drop in current efficiency of40% during regeneration of the electrolyte, the efficiency was 48% at0,20 M of Cr VI.

[0111] However, it was observed that by letting the used untreatedsolution rest for 2 days, there was a higher current efficiency, 62%with 0.227 M of Cr VI. It seems that by letting the electrolyte rest fora certain period of time, the current efficiency would increase duringthe regeneration. This increase in efficiency can be explained by thefact the residual Cr VI in the solution oxidizes the organic moleculescontained therein even at room temperature. A decrease in the Cr VIconcentration determined by titration, after letting the electrolyterest for a certain period of time, has confirmed such hypothesis of anoxidation. This oxidation can be combined with the evaporation of someorganic products dissolved in the electrolyte, like, for example, theextraction solvent (petroleum ether).

[0112] Treatment by extraction with valeric acid

[0113] Valeric acid is a five carbon atoms organic acid. The degradationproducts in the solution mostly consist of shorter chain acids andshould have an affinity for valeric acid. By using valeric acid asextraction solvent after the oxidation reaction, two extractions werecarried out on the electrolyte with 100 ml of valeric acid. Then, asecond series of extractions were performed with two portions of 100 mlof petroleum ether at 35-60° C. to remove the traces of valeric acid inthe electrolyte.

[0114] With this treatment, the total amount of organic carbon (TOC) wasreduced from 3,95 g/l in the untreated electrolyte to 2,031 g/l in theelectrolyte treated with valeric acid.

[0115] To determine the true efficiency of such a treatment, aregeneration of the electrolyte was carried out. A current efficiency of45% was obtained with 0,1640 M of Cr VI.

[0116] This was clearly insufficient for use with electrolysisprocesses.

[0117] Treatment with pelargonic acid

[0118] With the same basic premise in mind as for the treatment withvaleric acid, extractions were carried out on the electrolyte with twobatches of 100 ml of pelargonic acid. These extractions were followed byother extractions with two batches of 100 ml of petroleum ether at35-60° C. The TOC obtained was of 2,261 g/l as compared to 3,95 g/ml ofthe untreated electrolyte. The current efficiency of 48% was obtainedwith 0,20 M of Cr VI.

[0119] This treatment is clearly not efficient enough to justify itsuse.

[0120] Thermal treatment (invention)

[0121] By heating the used untreated solution, it was discovered thatthe oxidation process could be accelerated when the solution was allowedto rest for 2 hours. After the reaction was completed, the electrolytewas heated and kept at 110° C. for 90 minutes. The above-mentioned timedoes not include the time it took for the solution to reach 110° C. Indoing so, oxidation with the residual Cr VI was favored up to thedecomposition of the products into CO₂, and the solvent and some organiccompounds were eliminated by evaporation. Thermal treatment gave thelargest decrease in TOC. After this treatment, there was only 1,67 g/lof TOC. Such corresponds to a decrease in 64% TOC. The currentefficiency of the regeneration was approximately 78% for a solution with0,21 M of Cr VI, as compared to the current efficiency of 78% for a<<new>> solution. Heating is therefore important for the treatment ofthe electrolyte.

[0122] Table 3 summarizes the data obtained during the determination ofthe treatment to be used. TABLE 3 Current efficiency obtained with thedifferent treatments of the electrolyte Peak Concentration of CrCumulative current Trial Type of Treatment VI (M) during electrolysisefficiency (%) E-02 (new solution) Reference 0.2207 79.36 E-03 (newsolution) Reference 0.2227 77.19 E-04 (new solution) Reference 0.177089.12 E-06 (used untreated) Reaction 0.2146 44.89 E-07 (used untreated)Reaction 0.2437 50.72 E-09 (used untreated) Reaction 0.2023 53.43 E-10Heating 0.2210 78.06 E-12 Reaction 0.2307 41.05 E-13 Heating 0.276087.20 E-14 Pelargonic 0.2026 47.65 E-15 Pelargonic 0.2267 62.02 E-17Heating 0.2493 37.81 E-18 Reference 0.2666 65.57 E-19 Valeric 0.168039.51 E-20 Valeric 0.1600 49.54 E-21 Reaction 0.2140 44.70 E-22 Reaction0.1933 40.17 E-25 Heating 0.2093 73.52 E-32 Heating 0.2653 66.89

[0123] Tests were also carried out with a FM01 cell. The referencesolution used in this cell had a current efficiency of 66% with 0,267 Mof Cr(VI). The solution that was used for the chemical reaction and wastreated by heating, had a current efficiency of 67% with 0,265 M ofCr(VI). Thermal treatment is therefore very efficient.

[0124] Table 4 summarizes the results obtained with the differentmethods of treating an electrolytic solution. TABLE 4 Total organiccarbon concentration Treatment (g/l) Untreated 3,965 Extraction withvaleric acid 2,031 Extraction with pelargonic acid 2,261 Cooling andfine filtration 3,550 Activated charcoal 3,503 Heating 1,674

[0125] Optimization of the treatment

[0126] An optimization of the treatment was done with the two variablesthat had to be optimized, viz. time and temperature. Time andtemperature are two factors that are linked together. Therefore, the“surface response” software ECHIP was used to check their interaction.Another parameter to be optimized is the minimum Cr VI concentrationnecessary in the solution to be heated in order to considerably decreasethe level of TOC in the electrolyte.

[0127] With ECHIP, an experimental testing plan was put forward. Toperform this testing plan, oxidation of oleic acid was first carriedout. Then, the electrolyte was divided into several 20 ml portions and atest was carried out on every one of these portions. The analysis of CrVI allows to track the progression of the oxidation of the impurities.The results that were reported show that the higher are the temperatureand the time, the higher is the degree of progression of the oxidation.The results also show that temperature is the parameter which influencesthe treatment the most.

[0128] Table 5 gives an overview of the tests carried out during theexperimental testing the ECHIP software. TABLE 5 Treatments suggested byECHIP and results obtained during system optimization Trial Time (min)Temperature (° C.) Cr(Vl) (M) 4 75.0 120.0 0.0087 11 28.3 93.3 0.1820 575.0 40.0 0.2047 2 5.0 80.0 0.1853 10 75.0 93.3 0.0900 2 5.0 80.0 0.18205 75.0 40.0 0.2280 8 28.3 40.0 0.1733 3 40.0 120.0 0.0080 1 5.0 120.00.573 3 40.0 120.0 0.0027 7 51.7 40.0 0.2040 6 5.0 40.0 0.2400 1 5.0120.0 0.0780 4 75.0 120.0 0.0020 9 75.0 66.7 0.1240 12 70.0 60.0 0.140713 20.0 60.0 0.1800 14 20.0 90.0 0.1567 15 70.0 90.0 0.0607

[0129] Cr VI Concentration (inferior limit)

[0130] Tests were carried out in order to evaluate the impact of ahigher concentration of Cr VI on the efficiency of the regenerationcurrent and also to evaluate the minimal amount of Cr VI that thesolution must contain to have a good current efficiency after treatment.

[0131] In order to determine the influence of the Cr VI concentration onthe quality of the treatment (TOC level and regeneration currentefficiency), different amounts of a 0,6 M Cr VI solution were added to astarting solution containing 0,2 M of Cr VI, in order to increase thelevel of the Cr VI concentration. Subsequently, the solution was heatedat 120° C. for 90 minutes.

[0132] The obtained results show that the level of TOC decreases whenthe Cr VI concentration in the solution increases. However, it isimportant to note that in order to have a 0,25 M concentration, it wasnecessary to add 3,58 ml of a 0,6 M solution to the 20 ml sample of 0,2M. Such corresponds to 20% of the volume of the initial solution.Regeneration of this solution was carried out and the current efficiencywas increased by approximately 2% only.

[0133] Table 6 summarizes the obtained results. TABLE 6 TOCconcentration in the electrolyte as a function of the Cr VIconcentration Volume of added Concentration of TOC of the Samplesolution (ml) Cr VI electrolyte (g/l) Start (ref.)- 20 ml 0.00 0.19071.990 1 3.58 0.2500 0.845 2 10.14 0.3400 0.198 3 16.69 0.3800 0.213

[0134] The above study of different treatments for the oxidative acidsolution, has permitted to optimize the treatment. To have an efficientregeneration of the solution, heating of the same in the presence of CrVI is important. Heating in the presence of Cr VI permits to oxidize aportion of the organic compounds that are present in the solution and toevaporate volatile molecules, particularly the extraction solvent.Therefore, the current efficiency was found to increase to a value ofabout 80%. This value is very close to the maximum current efficiency(82%) obtained with a new solution. Optimization of the treatment by theECHIP software has permitted to assert that a maximum amount ofimpurities is oxidized when heating is performed as long as possible ata high temperature (110° C.). It also showed that the temperature playsa more prominent role than the duration of treatment.

[0135] In summary, regeneration by thermal treatment of the electrolyteconstitutes a key feature of the method according to the invention.Because of the nature of the products to be removed, the regenerationstep was far from being obvious. In fact, carboxylic acids and diacidshave a very high affinity for the aqueous oxidative phase. Moreover,solvents capable of extracting these compounds while remaining stablechemically and electrochemically in the medium, are practicallynon-existent.

Example 3 Electrolytic regeneration of a Ce IV solution for theelectrosynthesis of chlorobenzoguinone

[0136] The electrolyte used to catalyze the oxidation of chloroanilineto chlorobenzoquinone is the Redox couple Ce III/Ce IV.

[0137] This series of experiments was carried out in order to determinewhich one of the known treatments would be the best one for thepurification of the electrolyte before its regeneration in theelectrolysis cell. The three treatments selected for the purpose ofcomparison were the following: an addition of an excess of Ce IV to theelectrolyte before regeneration, the use of a more efficient solventduring the extraction step, and an improvement in the filtration step.

[0138] Addition of an excess of Ce IV to the electrolyte beforeregeneration

[0139] The idea behind using this treatment is to have a currentefficiency differential similar to that of a non-used solution at thebeginning of the regeneration of the solution in the electrolysis cell.

[0140] 200 ml of a solution containing 0,424 M Ce IV and 3,5 Mmethanesulfonic acid (84,8 mmoles of Ce IV), and 140 ml of a 1M solutionof Ce III were added to 1160 ml of used electrolyte solution. The totalvolume of the resulting mixture was 1500 ml. The mixture was then heatedat 60° C. for 30 minutes. Then the solution was regenerated in theelectrolysis cell.

[0141] The current efficiency at the beginning of the regeneration was83% for the solution with a Ce IV addition. This result is very similarto what is obtained for an unused solution, which is approximately 82%.The current efficiency values remained similar as the regeneration tookplace, that is, the Ce III concentration decreased and the Ce IVconcentration increased.

[0142] Use of a more efficient solvent during the extraction step

[0143] This test has permitted to determine whether the impurities thatwere present in the solution were in a dissolved or insoluble state.

[0144] 1250 ml of the used electrolyte solution were extracted in fiveconsecutive steps by using 250 ml of dichloroethane (for a total of 1250ml of dichloroethane) at 60° C. After the phase separation, theelectrolyte was regenerated electrochemically.

[0145] The differential current efficiency for this treatment was 67%.This is an improvement over the current efficiency of an untreatedsolution, which was 59% only. This increase in the current efficiencyshows that some of the impurities which affect the current efficiencyare in a dissolved state. However, the current efficiency resulting fromthe treatment is not sufficiently high to compare it advantageously tothe results obtained with a Ce IV addition. Thus, a treatment using amore efficient solvent does not represent an improvement substantialenough to justify its use.

[0146] Filtration step

[0147] This other test has permitted to confirm the results obtainedduring the extraction step by using a more efficient solvent. If theimpurities which decrease the current efficiency were in an insolublestate after the treatment, there should have been a substantialimprovement in the current efficiency in the electrolysis cell. If theywere present in a dissolved state, the treatment should not affect thecurrent efficiency in the electrolysis cell.

[0148] As soon as the electrolysis solution was cooled down to roomtemperature, it was subjected to a first filtration under vacuum using aWhatman® 934-AH glass fiber filter of 1,5 microns, and then a secondfiltration with glass fiber filter of 1 micron. The solution was thentransfered to the electrolytic cell for its regeneration.

[0149] The filtration step of the electrolytic solution had no impact onthe current efficiency 5 since the starting current efficiency for thefiltered solution was of 58%, while the starting current efficiency foran untreated solution was of 59%. This confirms the results of theextraction treatment made with dichloroethane, which indicated that theimpurities were present in a dissolved state.

[0150] Thus, in this example, three treatments were studied to determinewhich one would be the most efficient one to increase the currentefficiency of a used electrolytic solution. The most efficient treatmentwas the one which involved an addition of Ce IV in the electrolyticsolution. This treatment has allowed to obtain an initial currentefficiency equal to the one of an unused solution. This treamentconsisted in adding, in this case, an equivalent of 15% of the initialelectrolyte volume. This treatment seems relatively rapid since, at 60°C., 98% of the Ce IV that was added, was consumed in 30 minutes.

1. A method for purifying a Redox mediator used in a chemical process ofoxidation of organic compounds before said mediator is regenerated in anelectrolysis cell, comprising: recovering the mediator in the form of asolution containing volatile, soluble and insoluble impurities;subjecting the solution containing the mediator to a thermal treatment,said treatment being carried out in a reactor that is maintained at atemperature that is high enough to allow oxidation of the impurities bymeans of the left-over unused catalyst present in the solution and tosimultaneously allow elimination of the volatile impurities present inthe solution; and filtering the solution that contains the mediator toremove the insoluble impurities therefrom.
 2. Method according to claim1, further comprising: adding to the reactor a given amount of themediator regenerated in the electrolysis cell in order to increase theamount of said unused mediator present in the solution and therefore tocomplete oxidation of the impurities.
 3. Method according to claim 1,wherein the thermal treatment is carried out at a temperature rangingbetween 40 and 120° C.
 4. Method according to claim 3, wherein thethermal treatment is carried out at a temperature ranging between 70 and110° C.
 5. Method according to claim 4, wherein the thermal treatment iscarried out at a temperature ranging between 90 and 100° C.
 6. Methodaccording to claim 5, comprising: adding to the reactor a given amountof the mediator regenerated in the electrolysis cell in order toincrease the amount of the said unused mediator present in the solutionand therefore to complete the oxidation of the impurities.
 7. Methodaccording to claim 1, wherein the Redox mediator is selected from thegroup consisting of Cr III/Cr VI, Ce III/Ce IV and mixtures thereof. 8.Method according to claim 5, wherein the Redox mediator is selected fromthe group consisting of Cr III/Cr VI, Ce III/Ce IV and mixtures thereof.9. Method according to claim 6, wherein the Redox mediator is selectedfrom the group consisting of Cr III/Cr VI, Ce III/Ce IV and mixturesthereof.
 10. Method according to claim 1, wherein the chemical processof oxidation is a process synthesis of p-tolualdehyde from p-xylene. 11.Method according to claim 8, wherein the chemical process of oxidationis a process synthesis of p-tolualdehyde from p-xylene.
 12. Methodaccording to claim 9, wherein the chemical process of oxidation is aprocess synthesis of p-tolualdehyde from p-xylene.
 13. Method accordingto claim 1, wherein the chemical process of oxidation is a processsynthesis of naphtoquinone or its derivatives from naphtalene or itsderivatives.
 14. Method according to claim 8, wherein the chemicalprocess of oxidation is a process synthesis of naphtoquinone or itsderivatives from naphtalene or its derivatives.
 15. Method according toclaim 9, wherein the chemical process of oxidation is a processsynthesis of naphtoquinone or its derivatives from naphtalene or itsderivatives.
 16. Method according to claim 1, wherein the chemicalprocess of oxidation is a process synthesis of chlorobenzoquinone fromchloraniline.
 17. Method according to claim 9, wherein the chemicalprocess of oxidation is a process synthesis of chlorobenzoquinone fromchloraniline.
 18. Method according to claim 1, wherein the chemicalprocess of oxidation is a process synthesis of azelaic acid andpelargonic acid from oleic acid.
 19. Method according to claim 8,wherein the chemical process of oxidation is a process synthesis ofazelaic acid and pelargonic acid from oleic acid.
 20. Method accordingto claim 9, wherein the chemical process of oxidation is a processsynthesis of azelaic acid and pelargonic acid from oleic acid.